1. Technical Field
The invention generally relates to image forming apparatuses such as laser printers. More particularly, the invention relates to a system and method for increasing resolution of a laser printer through the use of a digital pulse width modulator that clocks digital data specifying grayscale values of pixels to be printed to the laser on both the ascending and descending edges of the clock, effectively doubling the clock rate and thereby increasing the resolution of the printer.
2. Technical Background
A typical laser printer usually includes an electrostatic printing mechanism composed of a cylindrical drum having an electrically charged surface. Toner particles of opposite charge adhere to the drum. The image to be printed is formed on the drum by means of a laser beam directed toward the drum. Wherever the laser impinges on the drum, the drum surface is discharged, creating an area in which the charged toner particles will not adhere, corresponding to white areas in the image. Solid areas are represented by the charged areas of the drum, where the toner particles adhere. The laser, driven by a bitmap image signal composed of binary data, scans the drum line by line, emitting pulses that correspond to the white and black areas of the image. Subsequently, the image is printed to paper by transferring the toner on the drum surface to paper by means of a heating process. The laser has only two states, on and off, and, thus, is capable of rendering only black and white areas. This arrangement is well suited to printing of text, where the characters have sharp edges and the image typically only includes black text and white space. However, images, such as photographs, have fuzzy edges and gradations in tone. Producing a quality print of such an image requires that the printer be able to produce intermediate tones, or grayscale values. Generally, grayscale values are produced using halftones, in which different values are represented by dots of varying size spaced at varying intervals. Thus, for a laser printer to print halftones, the output of the laser must be modulated, enabling it to produce the variably sized and spaced dots that make up a halftone image.
A common way of driving the laser such that it can reproduce intermediate tones is to provide a pulse width modulator. Digital data specifying grayscale values of the pixels to be printed is supplied to the pulse width modulator, and the pulse width modulator outputs a signal that varies the width, and also the period of the laser pulses, producing variably sized and spaced dots. The prior art provides several examples of laser printers that include pulse width modulators: for example: S. Haneda, Y. Itahara, T. Hasabe, T. Niitsuma, Image forming apparatus with sub-pixel position control, U.S. Pat. No. 5,432,611 (Jul. 11, 1995), or T. Motoi, S. Haneda, Image forming method, U.S. Pat. No. 5,436,644 (Jul. 25, 1995), or Y. Itihara, S. Haneda, N. Koizumi, T. Hasabe, T. Niitsuma, Image forming apparatus with neighboring pixel control, U.S. Pat. No. 5,467,422 (Nov. 14, 1995), or S. Haneda, Y. Itihara, T. Hasabe, T. Niitsuma, Color image forming apparatus with density control, U.S. Pat. No. 5,473,440 (Dec. 5, 1995), or N. Koizumi, S. Haneda, Y. Ichihara, T. Hasabe, T. Niitsuma, Digital image forming apparatus using subdivided pixels, U.S. Pat. No. 5,486,927 (Jan. 23, 1996), or S. Haneda, Y. Ichihara, T. Hasabe, T. Niitsuma, Image forming apparatus that modulates image density data, U.S. Pat. No. 5,493,411 (Feb. 20, 1996) or S. Haneda, M. Fukuchi, T. Miwa, Image forming apparatus with edge point detector based on image density charge, U.S. Pat. No. 5,619,242 (Apr. 8, 1997). All of the previous examples describe an analog pulse width modulator circuit that includes a digital-to-analog convertor (DAC) and a comparator. The binary image data is converted to an analog signal. The image signal is compared with a reference signal to derive a pulse width-modulating signal. Such analog pulse width modulators, however, suffer several disadvantages. Due to their analog nature, they are inherently sensitive to noise and they are vulnerable to voltage drifts and temperature drifts, requiring frequent recalibration. Furthermore, they are implemented using discrete components, rendering them complicated and expensive. Thus, it would be desirable to provide a purely digital means of pulse width modulation that eliminated the disadvantages of the analog circuit.
Digital pulse width modulators are known in the art. Typically, these pulse width modulators include a pixel clock and a shift register. Each pixel of the image is represented by 8 bits. The 8 bits representing a pixel are loaded into the shift register in parallel. Subsequently, at the rising edge of each clock cycle, the data in the register is shifted by one value. Thus, one new value is output to the laser with every clock cycle. When all 8 bits have been output, the register is reset and reloaded with the data for another pixel. A deficiency of this type of arrangement is that the clock speed imposes an upper limit on the granularity, or resolution that can be achieved, thus limiting the image quality. J. Hewes, Method of increasing the grayscale resolution of a non-impact LED page printer, U.S. Pat. No. 5,105,202 (Apr. 14, 1992) describes such a system and suggests that resolution can be improved by increasing the data output of the shift register. However, no means for increasing the shift register's output is suggested. The practical maximum frequency for a pixel clock on a printed circuit board is approximately 100 mHz. Thus, in clocking data from the shift register only on the rising edge of the clock cycle, the maximum output of the shift register is approximately one new value every 10 ns, imposing an upper limit on the achievable resolution. Increasing the clock speed to achieve a greater output is not a practical or feasible solution.
Accordingly, it would be a significant technological advance to provide a simple, inexpensive way of increasing the output of a digital pulse width modulator in a laser printer, so that greater resolution is achieved, thereby providing a better quality output image. It would be highly advantageous to achieve such an improvement in resolution without resort to changing the clock speed.